The documentation page "Converting an existing website to a Joomla! website" does not give details of converting an existing website - it instructs the reader to manually recreate the website one page and menu item at a time, which is not a good suggestion for a website of any significant size.

What I expected the page to contain was any/all of:
* instructions on using built-in import functionality I'd overlooked
* instructions on using the API or SQL to build a bespoke importer
* an officially recommended importer extension

I could not find any of that information within docs.joomla.org though there appears to be plenty of forum posts with people asking for various bulk import solutions, so far most of which seem to be recommending dead/expired/paid extensions. :/

Precisely, so there is one way to structure the data, irrespective of what the source of that data is.

This is what I would expect to see on that page - a canonical example of programmatically creating a minimum/typical article record from a sufficiently generic data structure, an example of creating a category, and so on.

The page should explain how one ends up with three Joomla articles and two categories.

How the original website data is converted to that structure will depend on the source system, and could be considered beyond the scope of the article (which is fine), though it's probably helpful to include links to relevant export/api documentation or plugins for the most popular systems.

Basically:
* Converting from common CMS system, use these converters: [Wordpress] [Drupal] [etc]
* Converting from other systems, here's how to get started: [code examples]